The response of cells to heat shock has been well characterized at the cellular level. Hyperthermia induces a state of thermotolerance and a specific set of proteins (heat shock proteins). There is much evidence linking these two phenomenon in a causitive manner, but nothing is known about the molecular mechanisms by which cells become thermotolerant and what role heat shock proteins have in this process. We have shown that the rate of recovery of protein and RNA synthesis following heat shock correlates with cell survival and, by using levels of actinomycin D which specifically inhibit rRNA synthesis we have identified this as an essential process in the expression of heat resistance in thermotolerant cells. In this proposal we will: (1) Determine more precisely the mechanism by which actinomycin D is able to block the expression of thermotolerance by studying the inactivation of ribosomes by heat shock and comparing this to cell survival. Preliminary studies have implicated irreversible ribosome damage in cell death following heat shock. This will be investigated. (2) Determine the mechanisms underlying the more rapid recovery of rRNA synthesis in thermotolerant cells by using an in vitro transcription system and, (3) apply the information gained above to the RIF-1 in vivo/in vitro tumor system to see if actinomycin D can enhance the effectiveness of fractionated hyperthermia treatment of cancer. By using a molecular approach this study should lead to a clearer understanding of the mechanisms underlying thermotolerance and be directly relevant to the treatment of human cancer. Techniques: In-vivo labeling, polyacrylamide and agar gel electrophoresis, density gradient centrifugation, cell survival, in- vivo/in-vitro tumor systems, cell fractionation, in-vitro transcription systems, protein purification procedures.